Roll out the puff pastry on a lightly floured surface into a rectangle measuring about 40cm x 20cm. Trim all the edges with a sharp knife (this will help give a good rise), then cut into 8 equal rectangles. Place onto lightly greased baking sheets.

On each square, score a border 1cm from the edge, all the way round, taking care not to cut right through the pastry. Brush the surface with beaten egg. Bake for 20-25 minutes until risen and golden brown.

Meanwhile, melt the butter in a large frying pan and sauté the mushrooms for 3-4 minutes. Remove from the heat and cool.

When the pastry tarts are cooked, carefully remove the central rectangle from each one (these are the lids). Mix the crème fraiche and chives or parsley into the mushrooms and season to taste. Spoon into the cases and pop the lids on top. Warm through in the oven for a further 4-5 minutes, then serve.